Impedance control devices, also called tuners, are devices of which the impedance, presented to the outside world, can be changed. This is done by either manually changing a device property (e.g. a manual tuner, see e.g. http://www.maurymw.com/MW_RF/Manual_Tuners.php) or by changing a property via an electronic means (e.g. an automated tuner, see http://www.focus-microwaves.com). The device typically has one port or two ports, but can in principle have more ports, e.g. a tuner for differential devices. Via the port these devices are connected to the outside world and provide a controllable impedance to the device under test (DUT). In most cases, a port is a physical connector through which the impedance control device can be connected to another device. However, the port does not need to be limited to a connector. The port defines a boundary between the impedance control device and the outside world. Amongst other things, a port can be a pad of an integrated circuit (IC) or a location on a line on a PCB. The impedance range the impedance control device can provide depends on the physical properties of the device.
Impedance control devices are well established in source- and load-pull measurement set-ups or measurement systems. These set-ups are used to determine the impedances to be presented at the input and/or output of a device under test in order to optimize one or more of its performance characteristics, e.g. the delivered output power, power added efficiency and other. In this case the device under test is typically a transistor or an amplifier under test. These set-ups are also used to characterize the behaviour of devices, e.g. transistors, diodes, amplifiers, mixers etc. . . . under realistic test conditions or to verify and/or improve their model, used in computer aided engineering (CAE) tools.
The impedance control devices, which are presently used in commercially available source- and load-pull systems, are based on various techniques. As measurement means, these source- and load-pull systems use different types of measurement receivers: one or more power meter, spectrum analyzer, network analyzer, oscilloscope, . . . .
First, there are the passive impedance control devices. They are based on one or more moveable resonator or slug. These tuners usually are bulky due to the mechanical aspects, while shrinking in size with increasing frequency due to reduced wavelength. As such they take a lot of space in a measurement set-up. To move the resonator(s) or slug(s) automatically, tuners contain step motors. As these parts are moving to synthesize a new impedance, the tuners can cause vibrations in the measurement set-up. This is typically a problem for on-wafer measurements. The use of pin diodes, positioned at different positions of transmission line stubs, has been an alternative to synthesize impedances in a passive way. This approach results in smaller form factors, eliminating the step motors, but is presently limited to power levels up to approximately 35-40 dBm. The working principle is based on creating reflections on a transmission line at different positions by turning on or off the pin diodes at these positions. As such the size is dependent on the frequency range. Pin diodes can be switched on and off very fast. Consequently impedances can be tuned fast.Secondly, there are the active tuners. Two types of active tuners do exist: closed loop and open loop. With the closed loop approach, the tuner senses the output power of the device under test (DUT), amplify or attenuate it and shift it in phase and re-inject this signal towards the device under test as reflected wave. Meanwhile proper selection in topology and narrowband filtering in the loop minimizes the risk of oscillation. These set-ups usually are bulky because the couplers, amplifiers, attenuators, filters and phase shifters are connectorized devices.With the open loop approach power is actively injected towards the DUT output in a phase coherent way with the source that provides the input signal to the device under test. This can be realized in different ways, e.g. by splitting the input source, followed by amplifying or attenuating and phase shifting it (as in the presentation “Active and passive load-pull systems: from the basic to the future of variable impedance device characterization”, A. Ferrero et al, PAF, pp. 13-14, IMS 2005 Workshop WSG), or by using a second source which is controllable in amplitude and phase and phase locked to the source at the input (see “High power active harmonic load-pull system for characterization of high power 100-watt transistors”, Z. Aboush et al., EUMC 2005, Proc. Vol. 1). For both approaches, the signal injected back to the device under test is amplified or attenuated and controlled in phase compared to the signal that comes out of the device under test. In this way synthetically different impedances can be synthesized. A similar approach can be used to synthesize impedances at the input of the device under test, typically at harmonics of the input source. Also this set-up is bulky requiring splitters, possibly a second source, amplifiers, phase control, possibly filters etc. . . . . .
Further impedance control devices are available which operate based on a combination of passive and active tuning. Reference is made to the web sites of Focus Microwave (www.focus-microwave.com) and HFE (www.hfemicro.com).
Commercially available set-ups, provided with an impedance control device, minimally contain a source to stimulate the device under test, the DUT itself, followed by a tuner and a means (e.g. a power meter) to measure the power transmitted by the DUT under different impedance conditions, as illustrated in FIG. 1.
Further extended set-ups also use power measurement capability at the input to measure input power, possibly in combination with a source tuner and the capability to measure reflected power at the input, a spectrum analyzer at the output to perform frequency selective power measurements and to monitor stability. Another exemplary set-up is shown in FIG. 2.
If one wants to measure more information at the DUT, it is possible to use a vector network analyser, an oscilloscope or a receiver with similar capabilities in combination with signal separation hardware that can detect samples of the incident and reflected waves (or a combination thereof).
In set-ups to measure the incident and reflected waves or a combination thereof, typically in a frequency-selective way, the signal separation hardware can be put (see FIG. 3) outside the combination of device under test and tuner (after the DUT and tuner) or between the DUT and the tuner (FIG. 4). With the first configuration (FIG. 3) one needs to use the S-parameters of the tuner to properly de-embed the measurements up to the device under test as the impedance is being changed. With the second configuration (FIG. 4) the incident and reflected waves or a combination thereof are always measured at the DUT, independently of the tuner position. One selects signal separation hardware that minimizes the losses between the DUT and the tuner, as the losses reduce the coverage area of the Smith chart.